Facile and Scalable Synthesis of “Caterpillar-like” ZnO Nanostructures with Enhanced Photoelectrochemical Water-Splitting Effect

Abstract

In an effort to replace conventional multistep seeding methods to produce multidimensional ZnO nanostructures with high spatial occupancy of nanowires (NWs), a rational and facile synthesis protocol for "caterpillar-like" branched ZnO nanofibers (BZNs) is reported. This process combined the scalable forcespinning technology with a hydrothermal process to efficiently develop BZNs to substantially enhance surface area and roughness factor. Specifically, unbranched ZnO nanofibers were first prepared by spinning polyvinylpyrrolidone (PVP)/zinc nitrate hexahydrate (Zn(NO 3 ) 2 •6H 2 O) composite fibers, followed by calcination at 500 °C. These fibers were deposited on flat substrates to serve as nonwoven netlike seed sites for hydrothermal growth of ultradense and uniform NW branches to form "caterpillar-like" BZNs. Their usage as photoanodes in photoelectrochemical water-splitting cells was evaluated, resulting in a photon-to-hydrogen conversion efficiency of 0.165%, an enhancement of 147% when compared to ZnO NW arrays with similar dimensions. An enhanced light-harvesting process coupled with a facile and scalable synthetic procedure is presented.

Recent Progress of Polyurethane-Based Materials for Oil/Water Separation

“…With advantages of low cost and high e±ciency, centrifugal spinning technology has been developed from the initial preparing arti¯cial hair to the recent fabricating nano¯ber membranes. 55,56 Ou et al 57 reported for the¯rst time on the fabrication of a robust, thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel-coated PU membrane by centrifugal spinning for e®ective separation of oil/water emulsions. The reason of selecting thermoplastic PU was its high tensile strength, wear resistance and good°exibility over a wide temperature range.…”

Branched nanostructures for photoelectrochemical water splitting

“…22 Material chemistry was tuned to yield secondary branches with high length-to-diameter aspect ratio 1·45 at 1·0 V against RHE (0·9 from the P-ZnO NWs/Au, 0·7 from the B-ZnO NWs and 0·5 from P-ZnO NWs) PCE = 0·52% @ 0·76 V against RHE (0·30 from the P-ZnO NWs/ Au, 0·24 from the B-ZnO NWs and 0·20 from P-ZnO NWs); IPCE = 5·3% @ 550 nm and 1·23 V against RHE (2·3 from P-ZnO NWs/Au, 0 from the B-ZnO NWs and 0 from P-ZnO NWs) 33 . When illuminated under AM 1·5G simulated light, the parental ZnO NFs produced only a small photocurrent density of ~0·001 mA cm −2 from −0·5 to +1·0 V and 0·089 mA cm −2 at +1·2 V possibly due to limited surface area, inefficient charge separation as well as the absence of effective electron transport paths between the NFs and from the NFs to the substrate.…”

“…Some merits of such caterpillarlike, hierarchically branched ZnO nanostructures responsible for the improvement of PEC efficiency are discussed in the original paper. 22 In spite of a validated improvement of PEC performances using the caterpillar-like branched ZnO nanostructures, the inherent large band-gap of ZnO restricts the visible light absorption in solar spectrum. Therefore, further improvement is envisaged by substitutional doping, noble metal coating and sensitising.…”

“…23 Although branched ZnO nanotetrapods and caterpillar-like branched ZnO nanostructures have been employed as photoanodes in PEC water splitting, ZnO nanoforests have not been utilised to build a 3-D interconnected conductive matrix for solar-tohydrogen production yet. 21,22 To fill the knowledge gap and discover more potentials of ZnO nanoforests in energy conversion and storage, three typical architectures, that is, ZnO NW arrays, brush-like nanoforest and willow-like nanoforest corresponding to samples H, HA5 and HAP3 in Figure 5, respecitively, were evaluated as photoanodes in PEC cells with the same footprint of 0·7 cm 2 from PEC water splitting for hydrogen generation ( Figure 6). Figure 6(a) shows the linear sweep voltammograms recorded from −0·5 to +1·2 V (against Ag/AgCl) on these three ZnO nanoarchitectures under irradiation (100 mW cm −2 ) as well as the NW arrays in the dark for comparison.…”

Compound Homojunction:Heterojunction Reduces Bulk and Interface Recombination in ZnO Photoanodes for Water Splitting

“…Charge recombination arises from poor conductivity, low hole mobility (1–15 cm 2 V −1 s −1 at 300 K), and sluggish oxygen evolution reaction kinetics . Attempts to improve charge separation and transfer have been made, such as through the use of heterojunctions, multi‐dimensional nanostructures, and oxygen evolution catalysts . However, it is still challenging to simultaneously reduce the bulk and interface recombination losses.…”